Hypoxia-inducible factor-2α and TGF-β signaling interact to promote normoxic glomerular fibrogenesis

Hypoxia-inducible factors (HIFs) are transcription factors consisting of an oxygen-sensitive alpha-subunit binding to a stable beta-subunit. HIFs regulate multiple signaling pathways that could contribute to fibrogenesis, supporting their potential role in hypoxia-mediated renal fibrosis. We previously reported that HIF-1 is upregulated and required for transforming growth factor (TGF)-beta induction of collagen in renal tubular cells. Here, we performed in vitro and in vivo studies of potential glomerular crosstalk between TGF-beta and normoxic HIF signaling. HIF-alpha has two major isoforms, HIF-1 alpha and HIF-2 alpha with different target gene sets. In cultured human mesangial cells, TGF-beta(1) treatment increased both HIF-1 alpha and HIF-2 alpha expression in normoxia. TGF-beta(1) did not increase HIF-1 alpha/2 alpha mRNA levels nor decrease the rate of protein degradation, suggesting that it enhances HIF-1 alpha/2 alpha expression through translation. TGF-beta receptor (ALK5) kinase activity was required for increased, TGF-beta -stimulated HIF-alpha expression in response to TGF-beta, and inhibiting PI3-kinase markedly decreased HIF-alpha expression. Blocking HIF-1 alpha/2 alpha expression using siRNA decreased basal and TGF-beta 1-stimulated type I collagen expression, while overexpressing nondegradable HIF-alpha increased the collagen response, with HIF-2 alpha being significantly more effective than HIF-1 alpha. In adriamycin-induced mouse glomerulosclerosis, HIF-2 alpha target genes were upregulated in sclerosing glomeruli. Taken together, our data demonstrate potential signaling interaction between TGF-beta and HIFs to promote renal fibrogenesis in normoxia and suggest that the HIF-2 alpha isoform is more important during glomerulosclerosis.